Editmesh: Toggle between all edge-loop boundaries when selecting?
It selects connected edge loops which are not connected to anything else (boundary edges):



This only affects cases where the loop is on the edge of a non-closed part of the mesh. For example, if you try this on an extruded grid (with thickness), nothing will happen as there are no boundary edges on a manifold mesh. 

It should also be noted that this doesn't seem to work with Shift.